Metabolism is a term that is used to describe all chemical reactions involved in maintaining the living state of the cells and the organism. Metabolism can be conveniently divided into two categories:

• Catabolism – the breakdown of molecules to obtain energy
• Anabolism – the synthesis of all compounds needed by the cells

Metabolism is closely linked to nutrition and the availability of nutrients. Bioenergetics is a term that describes the biochemical or metabolic pathways by which the cell ultimately obtains energy. Energy formation is one of the vital components of metabolism.

What does your metabolism do?

Your metabolism never stops, even when your body is at rest. It constantly provides energy for basic body functions, such as:

• Breathing.
• Circulating blood.
• Digesting food.
• Growing and repairing cells.
• Managing hormone levels.
• Regulating body temperature.

What is the basal metabolic rate (BMR)?

Your basal metabolic rate (BMR) refers to the minimum number of calories your body needs to function while you’re resting. This amount varies from person to person. Your BMR fulfills 60% to 70% of the energy your body uses. Rapid weight loss and aggressive calorie restriction decrease your BMR — this is one reason why weight loss is usually not linear and may stall at some point.

How does the body use the rest of its energy?

Your body uses about one-tenth of its energy to process food into fuel. The remaining energy fuels your physical movement.

How does metabolism affect weight?

Many people blame metabolic problems for weight struggles. But your metabolism naturally regulates itself to meet your body’s needs. It’s rarely the cause of weight gain or loss. In general, anyone who burns more calories than they take in will lose weight.

What’s the difference between a fast metabolism and slow metabolism?

Someone with a fast metabolism or fast BMR burns a lot of calories even while at rest. If you have a slow metabolism or slow BMR, your body needs fewer calories to keep it going.

A fast metabolism does not necessarily lead to thinness. In fact, studies show that people with overweight/obesity often have fast metabolisms. Their bodies need more energy to keep basic body functions going.

Factors that affect our BMR

Your BMR is influenced by multiple factors working in combination, including:

• Body size – larger adult bodies have more metabolising tissue and a larger BMR.
• Amount of lean muscle tissue – muscle burns kilojoules rapidly.
• Amount of body fat – fat cells are ‘sluggish’ and burn far fewer kilojoules than most other tissues and organs of the body.
• Crash dieting, starving or fasting – eating too few kilojoules encourages the body to slow the metabolism to conserve energy. BMR can drop by up to 15 per cent and if lean muscle tissue is also lost, this further reduces BMR.
• Age – metabolism slows with age due to loss of muscle tissue, but also due to hormonal and neurological changes.
• Growth – infants and children have higher energy demands per unit of body weight due to the energy demands of growth and the extra energy needed to maintain their body temperature.
• Gender – generally, men have faster metabolisms because they tend to be larger.
• Genetic predisposition – your metabolic rate may be partly decided by your genes.
• Hormonal and nervous controls – BMR is controlled by the nervous and hormonal systems. Hormonal imbalances can influence how quickly or slowly the body burns kilojoules.
• Environmental temperature – if temperature is very low or very high, the body has to work harder to maintain its normal body temperature, which increases the BMR.
• Infection or illness – BMR increases because the body has to work harder to build new tissues and to create an immune response.
• Amount of physical activity – hard-working muscles need plenty of energy to burn. Regular exercise increases muscle mass and teaches the body to burn kilojoules at a faster rate, even when at rest.
• Drugs – like caffeine or nicotine, can increase the BMR.
• Dietary deficiencies – for example, a diet low in iodine reduces thyroid function and slows the metabolism.

Thermic effect of food

Your BMR rises after you eat because you use energy to eat, digest and metabolise the food you have just eaten. The rise occurs soon after you start eating, and peaks two to three hours later.

This rise in the BMR can range between two per cent and 30 per cent, depending on the size of the meal and the types of foods eaten.

Different foods raise BMR by differing amounts. For example:

• Fats raise the BMR 0–5 per cent.
• Carbohydrates raise the BMR 5–10 per cent.
• Proteins raise the BMR 20–30 per cent.
• Hot spicy foods (for example, foods containing chili, horseradish and mustard) can have a significant thermic effect.

Energy used during physical activity

During strenuous or vigorous physical activity, our muscles may burn through as much as 3,000 kJ per hour. The energy expenditure of the muscles makes up only 20 per cent or so of total energy expenditure at rest, but during strenuous exercise, it may increase 50-fold or more.

Energy used during exercise is the only form of energy expenditure that we have any control over.

However, estimating the energy spent during exercise is difficult, as the true value for each person will vary based on factors such as their weight, age, health and the intensity with which each activity is performed.

How can I have a healthy metabolism?

These steps may benefit your metabolism:

• Don’t skip meals. Your metabolism quickly adapts and starts using fewer calories for body functions. If you restrict calories too much, your body starts to break down muscle for energy. A loss of muscle mass slows the metabolism.
• Fuel your metabolism with fresh fruits and vegetables, lean protein and healthy carbohydrates and fats.
• Strength train or do other weight-resistance type exercises to build muscles.
• Quit smoking. Your metabolism may slow down a bit, but you’ll lower your risk of cancer, heart disease and other problems.

When should I talk to a doctor?

You should call your healthcare provider if you experience:

• Extreme fatigue.
• Unexplained weight loss or weight gain.